Friday, 26 February 2016

METER IN METER OUT AND BLEED OFF HYDRAULIC CIRCUIT

METER IN METER OUT AND BLEED OFF HYDRAULIC CIRCUIT

We were discussing various types of hydraulics
valves used in hydraulic system in our previous post e.g. Servo valve and
proportional valve. Today we will be concentrated here with various types of
flow control hydraulic circuits usually used in various hydraulic units.

In order to control the speed of an actuator such as
hydraulic cylinder, hydraulic design engineer consider the flow control circuits
and these are basically three types. Meter-in circuit, meter-out circuit and
bleed of circuit.

Flow
control circuit: Meter-in

Meter-in flow control circuit represents the
controlling of fluid flow just before fluid enters to the actuator with the
help of flow control valve. In meter-in flow control circuit, flow control
valve will be installed in such a way that fluid flow to the actuator could be
controlled or we can also say that there could be a restriction in fluid flow
to the actuator.

Let us consider meter-in circuit for controlling the
flow of fluid and hence speed of actuator e.g. hydraulic cylinder in following
circuit. We can see here the bypass check valve that will force the fluid to
flow through the adjustable orifice before fluid enters to the actuator i.e.
hydraulic cylinder here.

Figure showing the meter-in flow control hydraulic
circuit, first circuit represent the rest position of cylinder while pump will
be in running condition. Second circuit represents the extension of hydraulic
cylinder.

Therefore, speed of an actuator will be controlled
here by controlling the flow of fluid before it enters to the actuators with
the help of flow control valve which will be basically an adjustable orifice
with bypass check valve as shown in above hydraulic circuit.

Let us consider, flow control valve is not provided
then in that case flow of fluid and thereby extension as well as retraction of
actuator i.e. cylinder will be carried out with unrestricted rate.

Movement of
actuator during extension and retraction will be controlled by providing flow
control valve as we have seen above. When fluid flow direction will be reversed
in that situation, fluid will flow through the check valve and hence will
bypass the flow control valve.

Meter-in flow control circuit will have quite
precise control if load is resistive load but if there will be overrunning load
then in that situation meter-in circuit will not be able to control the actuator
speed.

In case of overrunning load, actuator will move faster and hydraulic
circuit will not be able to fill it with hydraulic fluid and hence cavitation
phenomena will be possible over there.

Meter-in flow control circuit applications could be
seen in feeding welding machine, grinder table and also in milling machines.

Flow
control circuit: Meter-out

As we have discussed above, if load on actuator is
overrunning load in that situation meter-in flow control circuit will not be
able to control the speed of actuator because overrunning load on actuator will
make actuator rod i.e. cylinder rod to extend due to weight of vertical load
i.e. overrunning load.

Flow control valve will be installed on discharge
end or return side of actuator i.e. cylinder in order to control the discharge
of fluid flow. Meter-out flow control circuit will control the flow of fluid
leaving the actuator.

Figure showing below the meter-out flow control circuit,
first circuit represent the rest position while pump is in running situation. When
DC valve will be operated, we can see the second circuit to determine the fluid
flow to the cylinder through the check valve. Fluid will flow via check valve
by bypassing the flow control valve. When fluid will leave the cylinder, fluid
will not be able to flow via check valve and will be restricted by an
adjustable orifice.

Therefore actuator speed will be controlled in
meter-out circuit by restricting the flow of fluid leaving the actuator. Meter-out
circuit will work successfully with resistive load and also with overrunning
load or running away load because actuator will not be able to move faster than
fluid discharge it permits.

Bleed-off
flow control circuit

Bleed-off flow control circuit is also work successfully
only with resistive load because we are also controlling here the fluid flowing
to the inlet of the actuator.

Figure showing below the Bleed-off flow control
circuit, where we may see one needle valve installed at the inlet of the
actuator. Inlet of needle valve is connected with a line which is going to the
inlet of actuator i.e. cylinder. Outlet of needle valve will be connected here
with tank.

Second circuit in above figure indicates the extension
of hydraulic cylinder. Hydraulic fluid will flow to the inlet of actuator but
during the flow to actuator, excess flow of fluid will be bled off to the
hydraulic tank.